1. Field of the Invention
This invention relates to antiviral agents, in particular to compounds useful in the treatment of infections caused by Picornaviridae, such as human rhinovirus (HRV), and methods for their preparation. The invention also relates to the use of these compounds in the treatment of picornavirus infections and to intermediates useful in the preparation of these compounds. The compounds of the invention are especially suitable for use in the treatment of HRV and accordingly it will be convenient to describe the invention in connection with these viruses. However, it is to be understood that the invention is also applicable to other viruses of the Picornavirus family.
2. Description of the Related Art
Human rhinoviruses are a member of the genus Rhinovirus of the picornavirus family and are believed to be responsible for between 40 and 50% of common cold infections. Human rhinoviruses comprise a group of over 100 serotypically distinct viruses and accordingly antiviral activity for multiple serotypes and potency are considered to be equally important factors in drug design.
Two cellular receptors have been identified to which almost all typed HRVs bind. The major group, which comprises 91 of the more than 100 typed serotypes, binds to the intracellular adhesion molecule-1 (ICAM-1) while the minor group, which comprises the rest of typed serotypes with the exception of HRV87, binds to the low density lipoprotein receptor family of proteins.
Another genus of the Picornaviridae family is represented by the Enteroviruses. This genus includes polioviruses 1-3, coxsackieviruses A (23 serotypes) and B (6 serotypes), echoviruses (31 serotypes) and numbered enteroviruses 68-71. The clinical syndromes caused by enteroviruses include poliomyelitis, meningitis, encephalitis, pleurodynia, herpangina, hand foot and mouth disease, conjunctivitis, myocarditis and neonatal diseases such as respiratory illnesses and febrile illnesses.
Viruses of the Picornavirus family are characterized by a single stranded (+) RNA genome encapsidated by a protein shell (or capsid) having pseudo icosahedral symmetry. The surface of the capsid contains “canyons” which surround each of the icosahedral fivefold axes, and it is believed that the cellular receptors bind to residues on the canyon floor.
A hydrophobic pocket lies underneath the canyon within which a number of antiviral compounds are capable of binding, sometimes with consequential conformational changes. Some of these compounds have been shown to inhibit the uncoating of HRVs and, for some of the major receptor group viruses, inhibition of cell receptor binding has also been demonstrated. It has also been shown that when a compound is bound within the hydrophobic capsid pocket, HRVs are more stable to denaturation by heat or acids.
Examples of antipicornaviral compounds believed to act by binding within the hydrophobic pockets of the picornavirus capsid are described in U.S. Pat. Nos. 4,857,539, 4,992,433, 5,026,848, 5,051,515, 5,100,893, 5,112,825, 5,070,090, and Australian Patent No. 628172. One compound that has been the subject of recent human clinical trials is ethyl 4-[2-[1-(6-methyl-3-pyridazinyl)-4-piperidinyl]-ethoxy]benzoate, otherwise known as “Pirodavir”. (“Intranasal Pirodavir (R77,975) Treatment of Rhinovirus Colds” F. G. Hayden, et al., Antimicrobial Agents and Chemotherapy, 39, 290-294, 1995.)